Rolling bearings are generally made of bearing steel or case hardening steel which is to be carburized. Since rolling bearings are to be used with a shearing stress repeatedly imposed thereon under a high contact pressure, bearing steel is subjected to hardening and tempering, and case hardening steel is subjected to carburizing followed by hardening and tempering in order to secure a rolling fatigue life withstanding the shearing stress.
Compared with case hardening steel requiring carburizing, bearing steel is largely advantageous in terms of cost of heat treatment, reducing the total cost of production. However, bearing steel is disadvantageous from the standpoint of workability of the raw material and has an insufficient rolling life under a severe running condition. In order to overcome these disadvantages of bearing steel, techniques for improving the raw material workability to further reduce the production cost and for extending the rolling life have been proposed as disclosed, e.g., in JP-A-8-3689 and JP-A-8-53735 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, case hardening steel is, while economically disadvantageous, advantageous for its longer rolling life. In recent years, however, the working environment of bearings has been getting stricter. In particular, there has been a demand for extension of a life under contaminated lubrication as well as reduction of cost. Under these circumstances, the state-of-the-art case hardening steel cannot sufficiently meet the demands of life and cost.
The applicant previously proposed in JP-A-2-125841 a long-life rolling bearing made of an economical raw material which has a reduced Cr content (Cr is ready to precipitate into a carbide harmful to the life) and has added thereto as much Mn as the reduction of Cr to secure hardenability and which has an increased carbon content to improve productivity in heat treatment.
JP-A-8-4774 proposes increasing the carbon content of a raw material thereby to cut the cost of heat treatment, adding Si to the raw material, and subjecting the raw material to carburizing thereby to improve the life under contaminated lubrication.
According to the conventional techniques as taught by JP-A-8-3689 and JP-A-8-53735, somewhat extended durability of bearing steel has been attained, but no considerations has been given to the life under contaminated lubrication to cope with the recent environment of use of bearings.
The technique of JP-A-2-125841 aims at obtaining a rolling bearing having a long life under contaminated lubrication by retaining austenite in the surface layer in a proportion of 25 to 45% by volume. This can be achieved by setting the dissolved carbon content of at least the surface layer at 0.8% by weight or more so as to keep the retained austenite content within the above range. However, considering the case in which the carbonitrided surface is ground, the machinability (grindability) greatly varies depending on the surface nitrogen content, and the surface nitrogen content is decided by the alloying elements of the raw material and the conditions of heat treatment. From this viewpoint the technique of JP-A-2-125841 still has room for further improvement for reducing the total production cost, while paying due attention to efficiency of carburizing and grindability after carbonitriding as well.
Attention is not paid to the grindability after heat treatment in JP-A-8-4774, either. That is, considerations for not only grindability but materials, heat treatment, etc. are insufficient in making a reduction of production cost.
In other words, although the above-mentioned techniques have achieved an extended bearing life through improvement on bearing steel raw material that is economically advantageous, the life reached is still not equal to that of bearings made of carburized or carbonitrided case hardening steel. On the other hand, attempts to reduce the cost of case hardening steel that is to be carburized or carbonitrided have not succeeded in providing bearings competitive in price with normally hardened articles made of bearing steel.